Transformers. On the silver screen they disguise as cars. In hifi they appear in numerous junctions. Input trafos provide for level and/or impedance matching, galvanic isolation or balanced lines. Interstage transformers couple circuit stages in lieu of capacitors and/or function as phase splitters. Output transformers are de rigueur with valve amps to bridge hi-Ω power tubes and low-Ω loudspeakers. They also show up in DAC/preamps to generate symmetrical outputs with single-ended topologies. Transformers with extensive secondaries are even used as a type of volume control called TVC. With speakers transformers shows up in electrostats and ribbons. In powerline conditioners iron can show up as an isolation or balancing device.

But beyond step-up transformers in phono stages, we rarely see hifi circuits exploit magnetic gain. Usual gain devices are transistors, tubes or op-amps. But exceptions exist. The FirstWatt M2 and F6 power amps use small transformers for passive gain. Audio Consulting has its own version. In-depth canvassing of the worldwide web should scare up more yet the list will be short. Ask transformer detractors why so few. They'll likely point at limited bandwidth, phase shift, slow rise times and hysteresis effects whenever transformers are involved.

Into the narrow niche of yeahsayers now steps the Metrum Acoustics Aurix headphone amp from Dutch designer Cees Ruijtenberg. He's best known for his Octave and Hex zero-sampling converters using undisclosed high-speed non-audio ICs. For the Aurix he uses passive transformer gain. One FET ahead of the step-up iron acts as transimpedance coupler, one behind it as headphone driver. For the signal path that's it (or even less as Cees explains on the next page). So we're talking a conceptually pure minimalist circuit with potential promise in an application where overall gain needs (one-way headphones not multi-way speakers) are modest or nonexistent relative to modern high-output sources.

Early prototype with still single heatsink.

In fact high gain here means just 10dB. The amp's real job is to provide low-impedance current drive, not multiply source gain. The theme of simplicity continues with class A bias and neither local nor global feedback. Bandwidth is specified as 5Hz- 60kHz -3dB. Contrast this to the 600kHz-1MHz reach of competitors Questyle, Bakoon and Crayon. A golden spec surfaces with the 120dB S/N relative to 2Vrms out. On paper things again oxidize a bit for THD. Here it is 0.01% for 600Ω, a full 0.5% for 33Ω.
On specmanship more complex circuitry or hi-tech opamps could clearly gild these figures. One presumes that the Aurix was designed not just on the scope but also by ear. [Early show sighting at left.] Given the widespread acclaim for his converters, this would be successful MO for Cees Ruijtenberg.

What I didn't know? How many different headphones (from planar to dynamic, from high to low Ω and sensitivity) had Cees tried before signing off? Bakoon for example had to readjust their HPA-21's gain post production. At first it came on too fast to make many popular headphones quite unusable. Though it's not a proper tech term, I call it gain poisoning. Aurix promised not to suffer it. But the opposite perhaps? Did it have enough piss and vinegar to manhandle an HE-6? You know, the mighty twins of mojo and moxy? With a name recalling the recalcitrant Gauls of the Asterix and Obelix cartoons (they kept resisting Roman overthrow whilst hunting boar and imbibing the secret potion of druid Getafix), would Aurix be a strongman? Or more of a hapless village bard called Cacofonix?

A final look at the specs shows how voltage swing drops from 6Vrms into 600Ω to 2Vrms into 33Ω. This suggests high-ish output impedance and optimal power transfer into rarer high Ω loads, not low-Z planars. How would that manifest?